1. Field of the Invention
This invention relates to downhole seismic services and more particularly to a system and method for deployment, mounting, and coupling of seismic sensors downhole.
2. Description of the Related Art
Seismic sources and sensors are often deployed in wellbores for a variety of oilfield operations, including monitoring of injection well operations, fracturing operations, performing “seismic-profiling” surveys to obtain enhanced subsurface seismic maps and monitoring downhole vibrations. Such operations include slim-to large-diameter boreholes, vertical to horizontal wells, open and cased holes, and high pressure and high temperature wells. Downhole sensors are sometimes utilized in combination with other logging services, either wireline, coiled tubing-conveyed, or with pipe to provide additional reservoir information.
Seismic sensors deployed in wellbores are particularly useful to monitor fracturing and injection well operations, to generate cross-well information and to obtain seismic measurements over time, to obtain enhanced subsurface maps and to improve reservoir modeling. As used herein, seismic data refers to seismic signals generated by conventional surface or subsurface active seismic sources and to micro-seismic signals generated by formation fracturing. The majority of seismic data gathering is accomplished by wireline methods or by deploying seismic sensors such as geophones on coiled tubing or production pipe. Multi-component geophones are usually preferred for such applications. An example is the classical three (3) axis geophone which detects particle motion in three mutually orthogonal directions (x, y and z directions).
Coupling of the geophone/accelerometer elements to the formation via the casing/liner is a critical issue for the acquisition of microseismic energy around a sensor location. It is key to the processing of microseismic information that a particular microseismic event can be seen, and properly characterized, at multiple levels of the sensor string. Thus it is critical that sensor/formation coupling should be consistent from level to level. If the seismic event is not similar, in terms of amplitude, phase and frequency, from level to level, event identification and characterization (e.g. P-wave vs. S-wave) will prove difficult to impossible.
It is desired that the seismic sensors should be in a consistently coupled from level to level. Microseismic events are low amplitude and high frequency and are therefore extremely vulnerable to noise. Identification depends on being able to compare the signals from level to level, requiring that geophone placement is as consistent as possible.
Seismic coupling of the sensors to the formation is a major problem with prior art permanent and semi-permanent seismic sensors arrays for detecting seismic and microseismic events in deviated wellbores. As used herein, the term “deviated” is defined to mean all wellbores inclined from the vertical and includes horizontal wellbores. In vertical wellbores bow-spring technology, where the sensors are commonly held against the wall by the bow-spring, can be used to couple the sensors to a casing or liner that is coupled to the formation by cement. The bow-spring acts to decouple the sensors from the mass effects and vibration effects of the tubing, providing good frequency response. In deviated wellbores, bow-springs can not support the relatively heavy weight of the conveying tubulars. Difficulties in obtaining consistent sensor coupling and/or response can result. For example, if the sensor carrying bow-spring is oriented to the bottom of the hole, the weight of the tubing may be coupled to the sensor causing resonance/noise problems and reduced frequency response. If the sensor carrying bow-spring is oriented toward the high side of the hole, the sensor may be only lightly forced against the wall or it may not even contact the wall. The use of bow springs to couple multiple spaced apart sensors to the wellbore in deviated wellbores requires that the bow springs be oriented the same to provide substantially uniform coupling. Pipe or tubing that has been rotated during insertion in the deviated well bore may have latent rotational torque in the tubing causing rotational misalignment of initially aligned sensors. In addition, coiled tubing has a natural torque and tends to corkscrew in the wellbore providing unpredictable coupling.
When the wellbores are vertical and susceptible to cement injection, the sensors may be cemented in place to provide and effective acoustic coupling with the formation structure. However, seismic sensor coupling to the formation structure by means of cementing may be precluded in deviated, including horizontal, wellbores due to the type of completion used. For example, seismic acquisition may be desired in an open-hole section of a long horizontal wellbore.
Thus there is a need for an apparatus and method for deploying permanent seismic sensors in deviated wellbores and ensuring that the sensors are consistently seismically coupled to the wellbore.